Rumble is a mail server suite for SMTP (ESMTPSA), HTTP, POP3, and IMAP4v1 with an extensive C/C++ and Lua API for scripting. It comes with support for SSL/TLS, SQLite, and MySQL and has a Web-based administration and Web mail feature. Additional included modules feature greylisting, DNS blacklisting, SpamAssassin support, and dynamic performance scaling as well as a multithreaded structure that allows for clustering of multiple servers for one or all domains. With the Lua API you can create your own new server mailing lists, CRM software, e-marketing, and other extensions within minutes.

spmfilter is a mail filtering framework. It attempts to be a general
filtering framework for any purpose. The API enables spmfilter plugins
to access messages as they are being processed by the MTA. This allows
them to examine and modify message content and meta-information during
the SMTP transaction. Plugins are loaded at runtime and can be processed
in any sequence. The processing chain can also be altered by a single
plugin, for example if the plugin has to stop further processing.

Zimbra Virtual Appliance is a turn-key virtual appliance for the popular Exchange-replacement, Zimbra. The Virtual Appliance is configured according to Zimbra’s installation guide. It is compatible with VMware Server 2.0, ESX, and Fusion.

The Apache Mailet API is a Java API which allows
the rapid development and flexible deployment of
email processing functionality. A mailet is a mail
processing component. Though there are some
differences, a servlet as a HTTP processing
component is a useful analogy.

DJIGZO Email Encryption Gateway is a centrally managed email server that encrypts and decrypts incoming and outgoing email at the gateway level. Djigzo currently supports two encryption standards: S/MIME and PDF encryption. The built-in Data Leak Prevention (DLP) module can be used to prevent certain information from leaving the organization via email. DLP can be configured to filter on credit card numbers, bank account numbers, excessive amounts of email addresses or other personal information in one email message, and more. DJIGZO can be installed on most Linux and Unix based systems. Installation packages are available for Ubuntu, Debian, Red Hat and CentOS. A ready to run virtual appliance for VMware and Hyper-V is available. An add-on for BlackBerry is available.

Inspector is a little Python script for the
Postfix mail server that helps with detection of
NDR (Non Delivery Report) spam messages. It adds
a watermark to every email message being sent out
of the defined "mydomains", and implements a
mechanism for checking for the presence and
validity of such watermarks in every NDR message.
It means that you can check if the NDR message is
valid or not (i.e. it is NDR spam).

j-XChange is a pure Java implementation of the
entire Collaboration Data Objects (CDO 1.21)
library for accessing Microsoft Exchange Server in
a platform independent manner. This library will
allow interoperability with MS Exchange from all
non-Windows platforms that support Java. This
library is powered by j-Interop.

IndiMail is a messaging platform built using qmail and other messaging software. IndiMail has the ability to distribute the users in a domain across multiple hosts. IndiMail has multiple queues (called a queue collection), which can achiever very high concurrencies. The system IDs and configuration files used by IndiMail are all configurable through a set of environment variables, allowing you to customize IndiMail the way you want. IndiMail does not use hard-coded uid/gid in the executables, which makes it easy to distribute a binary package. IndiMail has features that any modern email server should have: SMTP, QMQP, QMTP, IMAP, POP3, DKIM, BATV, greylisting, filtering capability, and spam/virus control.